Remove the ribbon cables connecting the rail and battery casing to the motherboard. Unlock the cables by flipping the connector upwards with a spudger, then pull the cable out of the connector.

You may even choose to leave the rail connected, though it will be unwieldy.

Be careful not to lift the connectors in the wrong direction. The connection nearest to the shoulder buttons lifts up away from the rail (the black part), and the connection nearest to the rumble motor lifts in the opposite direction, up toward the rail (the thin grey part.)